This invention relates to the plastics packaging of glass optical fibres.
It is conventional practice to protect the surface of freshly drawn glass optical fibre with one or more plastics coatings. Such coatings are useful in protecting the glass surface from degradation by atmospheric attack, and also serve to provide protection from mechanical damage and to give additional strength for the package. On the other hand they are also liable to introduce problems associated with the inducing of microbending losses. An analysis of microbending loss problems in relation to optical fibres provided with single coatings and with double coatings is given in the paper entitled `Optical-Fiber Packaging and Its Influence on Fiber Straightness and Loss` by D. Gloge appearing in the Bell System Technical Journal, Vol. 54, No. 2 pp 245-262 (February 1975). The analysis of that paper indicates that a double coating consisting of a high modulus layer and a low modulus layer affords much better protection for a fibre than a single layer of either high or low modulus. Following from this, a conventional construction of plastics packaged fibre consists of a primary coating of a relatively low modulus material, such as a silicone rubber, enveloped in a secondary coating of a relatively high modulus material such as nylon. The application of such coatings to silica fibres designed for operation at a wavelength range in the region of 1.5 microns has produced fibres that can be satisfactorily cabled in a `tight` construction without introducing excessive microbending loss. In single mode fibre this microbending loss is revealed as a narrowing of the spectral transmission window of the fibre from its long wavelength end. This narrowing results from the fact that the loss induced by microbending is most severe for the most weakly guided light, that is the light of longest wavelength. For this reason the microbending loss effect is also sometimes known as the bend edge effect.
Subsequent to the development of the use of silicone rubbers for primary plastics coating optical fibres, it has been found that improved strength of primary coated fibre, in terms of fibre yield at a given proof strain, have been generally achieved when a urethane acrylate coating is substituted for the silicone rubber. However, when a urethane acrylate primary coated fibre is provided with a nylon secondary coating a significant bend edge effect is liable to occur with the application of the secondary coating or upon cabling. This finding is consistent with the Gloge's analysis, for the substitution of the urethane acrylate for the silicone rubber has resulted in changing the structure from a `hard-on-soft` (hard shell) package to a `hard-on-hard` package.
United Kingdom Patent Specification No. 2096353A describes an optical fibre that is plastics packaged with a triple plastics coating. The primary coating is a high modulus material in the range 200 to 400 kg/mm.sup.2 (c. 2.0 to 4.0 GPa), and is exemplified by a thermosetting resin, 200 microns thick, having a modulus of 200 kg/mm.sup.2. A phenolic resin is suggested as an alternative material. The secondary coating is a low modulus material in the range 0.2 to 0.9 kg/mm.sup.2 (c. 2.0 to 9.0 MPa), and is exemplified by a silicone rubber. This layer is 150 microns thick, and has a modulus of 0.35 kg/mm.sup.2. An acrylonitrile rubber is suggested as an alternative material. The tertiary coating is a high modulus material in the range 40 to 300 kg/mm.sup.2 (c. 400 to 3000 MPa), and is exemplified by nylon, 200 microns thick, having a modulus of 150 kg/mm.sup.2. Polytetrafluorethylene is suggested as an alternative material. The burden of the teaching accompanying the specific description is that the use of a particularly high modulus and relatively thick primary coating is required to stiffen the fibre so as to protect it from longitudinal compression produced by differential thermal expansion effects involving the tertiary coating.
We have found that these differential expansion induced longitudinal compression effects are not as significant as suggested, provided that effective use is made of the teaching regarding control of crystallinity in the extrusion of the nylon that is contained in the paper by S. R. Barnes et al entitled `Processing and Characterisation of Tight Nylon Secondary Coatings for Optical Fibres` given at the `Plastics in Telecommunications III` Conference, London, September, 1982, (Conference publication pages 15-1 to 15-12). Furthermore, we have found that, though a switch from a silicone rubber primary coating to one of a typical urethane acrylate, thereby increasing the modulus (tensile modulus at 2.5% strain) from the range of about 2.0-6.0 MPa to a few hundreds of megapascals, affords a useful increase in fibre strength; a further increase in modulus of the primary coating to about 1 GPa, afforded by using a high strength acrylate, through it may produce a further improvement in fibre strength, this is accompanied by a significant deterioration in optical properties of the fibre compared with those achieved with the lower modulus urethane acrylate. There is reason to believe that the use of a still higher modulus primary coating in the range 200 to 400 kg/mm.sup.2 suggested in GB No. 2096353A will produce a further deterioration in optical properties.